1. Field of the Invention
The present invention relates to a process and a system for multi-antenna transmission, in particular employed for providing down-link cellular coverage in a mobile communications network, and to a mobile terminal thereof.
2. Present State of the Art
As known, cellular-coverage mobile telecommunication networks have been rapidly evolving since their appearance on the mass market in the eighties.
Initially these networks mainly supported voice services, but today it is increasingly urgent, and now prevalent, the demand for data services. Technologic evolution is therefore stimulated by the constant demand for performance increase in terms of network transmission speed and capacity.
Standardization is now proceeding at a pace that was unusual until last decade for the telecommunications field, but now the stimulus created by the increasing band requirements of the various new data applications is leading to the introduction of new technologies offering better performance.
The increasing need for band, however, clashes with the limited radio resource. In fact, the radio frequencies used for mobile telecommunications services are limited and are rigidly assigned to a few operators, who cannot occupy frequencies they have no license for. Accordingly, it is not generally possible to increase frequency occupation in order to cope with the continuous rise in traffic.
The technical solutions which may be adopted to increase the networks' capacity, given a certain available radio frequency band, can be classified into two main categories:                techniques that allow greater frequency re-use, typically by increasing the number of cells (which thus become smaller and smaller, so much so that they are also known as “pico-cellular” coverages);        techniques adopting more efficient transmission and modulation methods, which allow transmitting greater quantities of data on the available radio frequency band.        
The number of cells and transmission efficiency are however both subject to physical and economical limits.
The limits of pico-cellular coverages are due to inter-cell interferences, which increase as cells get closer to one another; moreover, the increased number of radio base stations leads to considerably higher transmission costs, because of the connections that must be provided between the radio base stations and the transport network access points. In theory, the higher the number of cells in a cellular network, the greater the need for a complex transmission distribution infrastructure for connecting the radio base stations, which causes the cellular network to tend to resemble a fixed transmission network.
Network management costs also become higher because of the increased number of radio base stations.
Nevertheless, economic limits depend on considerations about economic advantage, and therefore are not absolute theoretical limits.
Technical limits, on the contrary, characterize each given technology in an absolute way.
The attainable transmission efficiency limits are determined by the fact that extreme modulations worsen the signal-to-noise ratio (SNR), until this becomes unsustainable for ensuring the proper operation of the system.
This is why intense research is being carried out to optimize transmission technologies, aiming at exploiting the available radio band at its most.
In order to offer a response to the above-described problems, MIMO techniques and techniques based on the use of “Vectorial Antennas” (also known as “Smart Antennas” or “multi-antenna system” or “multi-radiator system”) have been developed over time.
In particular, MIMO techniques express a concept which was first proposed by Arogyaswami Paulraj and Thomas Kailath in 1994: these are transmission/reception techniques based on the use of multi-antenna transmitters and receivers. In a typical embodiment thereof, MIMO transmission generates “N” signals on the same carrier, which signals are however “spatially” spaced from one another. Spatial separation is obtained by radiating signals from multiple antennas located in suitably different positions, and by picking up said signals by using a receiving system composed of multiple antennas, also spatially spaced from one another, on the hypothesis that the various propagation paths between the transmitting antennas and the receiving antennas have different transfer functions due to the multiple propagation paths.
The fact that a signal originated by a transmitting antenna propagates up to a receiving antenna through a certain number of multiple propagation paths is generally known as “multipath”.
This “multipath” propagation condition is normally found in cellular coverages within urban environments. Through the “multipath” effect, the signal that arrives at a point where there is a receiving antenna turns out to be the sum of a certain number of signals, one for each different path that the original signal may travel to arrive at said reception point. Said distinct signals essentially differ in the phase at which the carrier arrives at the reception point, due to the different length of each propagation path.
It is therefore clear that cases may arise wherein carriers are offset in a manner such as to be added together in a constructive way, or cases wherein carriers combine together in a destructive way. Such differences in the way that carriers combine together depend on the exact point where reception occurs, and the effects thereof may be very different (or even opposite) at distances comparable with the carrier wavelength, i.e. at distances of the order of centimeters for current radio mobile networks.
The above-described phenomenon, according to which the same signal may appear to be strong at one point and very weak at another very near point, is known as “multipath fading” or “fast fading”.
Referring back to the application of MIMO techniques, it can be observed that different signals originated at different points are characterized by different “multipaths”, resulting in different “multipath fadings”.
A typical way to handle “multipath fading” is to equip the receiver with at least two antennas arranged at a distance equal to approx. one fourth of the carrier wavelength, so that when the signal to be received is minimum at the reception point where one antenna is located, it will not be simultaneously minimum also at the reception point occupied by the other antenna.
In the case of a MIMO transmission, the different signals transmitted on the same carrier interfere with one another, of course, but they can be distinguished in reception by the diversity of the propagation paths traveled by them.
In MIMO transmissions, the various signals transmitted from different points appear to have different strengths (different “fadings”) at the various receiving elements of the receivers: in fact, in transceiving systems implementing MIMO techniques, also the receivers, not only the transmitters, must be multi-antenna systems.
A MIMO system therefore exploits “multipath fading” to distinguish among different signals transmitted on the same carrier but from distinct points.
In the case of a MIMO transmission, on the receiving side one can extract from each element of the receiving antenna system a signal given by a sum of all the transmitted signals, each transformed according to a different transfer function. In order to rebuild the original transmitted signals, it is therefore necessary to solve a system composed of as many equations as the number of receiving elements in the receiving antenna system.
It is clear that in a propagation environment not characterized by a sufficient and significant “multipath”, the various transfer functions would be all very similar and virtually equal to one another, so that the system would be impossible to solve.
From a physical standpoint, it can be stated that the more the transfer functions are similar to one another, the more the various signals will disturb one another, each one being noise for the others and contributing to making them unrecognizable.
In order to make the above-mentioned system easier to solve, so as to be able to discern among the various signals transmitted on the same carrier, MIMO techniques also make use of an additional device called “precoding”. “Precoding” provides for transmitting every single signal not only from a different antenna, but also with regulated phase displacement and amplitude. The objective of “precoding” is to make the system of equations to be solved in reception as simple as possible: in the most extreme and optimal case, a propagation situation is determined wherein the system to be solved in reception is characterized by a quasi-diagonal matrix, which corresponds to a case wherein each receiving antenna receives only one different signal which is significantly strong, the other signals being received very attenuated by each antenna.
In more sophisticated applications, MIMO techniques use a further device according to which every single signal is transmitted by more than one radiator and the various components (i.e. replicas of the same signal radiated by different radiators) also have different and adjustable phase and amplitude.
From a physical standpoint, as will be better explained below, the fact that every single transmitted signal is radiated by various radiators corresponds to introducing a directivity effect into the transmissions of the single signals.
It should however be noted that the “precoding” parameters must be continually updated, in that the “multipath fading” conditions change very rapidly: in fact, it is sufficient that the receiver (which, it must be reminded, is generally a radio mobile terminal) moves by just a few centimeters for the system to vary substantially.
“Precoding” must therefore be based on very sophisticated algorithms for updating the “precoding” parameters, and such algorithms must continually use feedback information communicated to the transmitter by the receiver.
Precoding efficiency is very important to determine the performance of the system: however, these algorithms may in general be very costly from a computational standpoint; for these reasons, much research activity is currently being carried out, aiming at optimizing this aspect of MIMO techniques.
In summary, a MIMO system owes its efficiency to the simple fact that multiple information streams can be transmitted on one same carrier, at the expense of the generation of much noise, which can however be handled by exploiting the “multipath fading” and said “precoding” techniques.
It is clear that “multipath fading” is indispensable for the proper operation of MIMO systems; consequently, said technology is applied to propagation situations wherein multiple paths are rather homogeneous, without a prevailing “direct” component of the signals, i.e. in complex coverage situations. In general, given the attenuation undergone by the signal because of multiple reflections, it is preferable that the transmission source emits signals at the highest possible power, so that it becomes less problematical to establish down-link MIMO transmissions, where the transmitter can be powered from the mains, than up-link transmissions, where the power resource is limited to the capacity of the battery of the mobile station, hereafter also referred to as (radio) mobile terminal or, more simply, terminal.
In short, it can be stated that MIMO techniques represent a promising solution to the problem of more efficient radio band utilization, and that the algorithms used for updating the “precoding” parameters constitute a key factor for obtaining the best performance from MIMO techniques applied to radio mobile systems.
A further known technique on which the present invention is based is the one using “Vectorial Antennas”, more commonly known as “Smart Antennas” or “Array Antennas”.
The so-called “Vectorial Antennas” are antennas composed of a plurality of radiating element (radiators), just like the antennas used for MIMO transmissions.
By suitably powering the different radiators, it is possible to obtain directive transmissions wherein pointing can be adjusted electronically without needing any physical movements of the antenna system.
“Vectorial Antennas” can also be used in reception and, through processes similar to those used in transmission (i.e. by dephasing the signals picked up by each element), it is possible to increase the gain of the overall system in some reception directions. However, the present invention is focused on down-link coverage, and therefore on transmitting systems.
In a transmission with a “Vectorial Antenna”, every single element radiates the same signal transmitted by the other antenna elements, but with an appropriately adjusted carrier phase displacement. Because of the different point of origin of the various transmissions, these components re-combine together with a phase displacement that varies according to the propagation direction: this occurs because the originally set phase displacement is added with the phase displacement given by the geometry of the transmitting system, which varies as a function of the transmission direction. It is thus possible to adjust the whole transmission in a manner such that there are transmission directions in which interference is maximally constructive and other transmission directions in which interference is maximally destructive. The system, considered as a whole, creates an adjustable directive antenna which, within certain angles, requires no mechanical movements, but only adjustments (executable via software) of the initial phase displacements.
The effect of forming a transmission lobe in a particular direction is known as “beamforming”.
It must be pointed out that the directive capacity of such antenna systems is strictly related to the number of antenna elements, since antennas made up of a larger number of elements are more directive (and therefore more directive antennas are bigger).
It is apparent from the above that there is an analogy between “precoding” and “beamforming”: in both cases, in fact, there is a transmission of a signal from multiple distinct points, wherein a different phase can be applied to each component. Notwithstanding said physical analogy, the two procedures, i.e. “beamforming” and “precoding”, are in practice two different procedures and are considered to be distinct techniques because they aim at different objectives.
The first procedure, “beamforming”, is generally implemented with a large number of radiators in order to obtain relatively narrow transmission beams. For good directivity, as aforesaid, it is necessary to employ a number of elements of the order of ten or, more appropriately, of a few tens. Furthermore, phase displacements calculation can be performed very easily in an open loop, the geometry of the “Vectorial Antenna” being known, as a function of the propagation directions in which one wants to obtain maximum radiation.
The transmission efficiency of a “Vectorial Antenna” is not affected by the fact that an antenna made up of multiple elements is used in reception.
The second procedure, “precoding”, can also be implemented with just a few radiators, even with only two antennas (this being the most widespread case).
In the “precoding” procedure, phase displacements calculation is generally performed in a closed loop in order to optimize the separation of two or more signals at a multi-antenna receiving device, which must therefore provide the transmitter with a continuous (or very frequent) feedback.
It is clear that “precoding” techniques require a number of computations that rapidly increases with the order of an antenna (i.e. with the number of elements that constitute the antenna).
MIMO techniques have already been employed in some radio mobile systems, and they will be used to a larger extent in the systems currently being standardized: however, due to the complexity of the art, it can be foreseen that their use will be limited to applications with a reduced order of antennas: this means that two or four transmitting elements and the same number of receiving elements will be mostly used.
It should be noted that a reduced order corresponds to reduced frequency re-use, thus posing a limit to the efficiency of the system as a whole.
The “Vectorial Antennas” technology, although already mature in many applications, has not yet been used in cellular coverage systems. The main cause of this is that the position of the mobile station within the cell could not be determined with the necessary accuracy, nor were any technologic standards available which allowed using sufficiently accurate information about terminal location; furthermore, the radio paths involved in transmissions on cellular coverage networks are normally reflected paths, and therefore it is almost always not true that it is convenient to transmit in the direction of the transmitter-receiver connecting line when transmitting towards a terminal.
These and other problems of variable nature have prevented the discovery of a feasible method for determining the pointing direction of the antennas in radio mobile networks, de facto preventing the use of “Vectorial Antennas” for cellular coverage applications.
The industry is currently deeply involved in the attempt at combining MIMO techniques with “beamforming”, which is typical of “Vectorial Antennas”, but all the solutions adopted so far have not proven to be sufficiently efficient and easy to implement.
In fact, as aforementioned, if the parameters required for determining the “beamforming” (i.e. the phase displacements to be applied to the components of one signal transmitted from multiple radiating elements in order to transmit it in a directive manner) are computed in a closed loop by using continuous feedback from the terminal, the computation complexity is such that it is necessary to limit the order of the antenna (i.e. the number of radiating elements). thereby also limiting the potential frequency re-use and transmission directivity.
Conversely, should one follow the route of determining the “beamforming” by computing the necessary phase displacement parameters in an open loop, this would lead to the problem of having to correctly choose the optimal pointing direction in a complex propagation environment characterized by many reflections.
This latter problem can be faced through some sort of implicit feedback from the terminal, the term “implicit feedback” referring herein to the fact that the terminal does not explicitly transmit a signal indicating how it is receiving, but such information is nonetheless obtained. It is in fact possible that at the transmission point there is also a directive receiver capable of “listening” to the signals coming from the mobile terminal, which therefore, in addition to acting as a receiver, must also transmit towards that radio base station, and of determining the optimal propagation direction by choosing the one in which reception strength is highest.
This mode has been recently set up at experimental level and is described, for example, in “A Proposal of DPC Beamforming for Open Loop Multiuser MIMO Transmissions”—Tomoko Matsumoto, Noriaki Miyazaki and Satoshi Konishi—KDDI R&D Laboratories, Inc.—IEEE ICC 2010 proceedings.
Although the set of processing operations necessary for implementing the technique proposed therein is still rather complex, it may yield substantial advantages: in fact, it is possible to implement higher-order antennas and to reach very interesting levels of spectrum utilization efficiency.
However, such a technique has a structural limit: in fact, it requires that the propagation channels be symmetrical, in that the transmission direction is determined depending on the quality of reception in the same direction.
This assumption is not generally true: it is in fact possible (and advantageous) that the coverages of the next-generation networks are characterized by the use of distinct radio stations positioned in different points to support up-link radio transmission (UL=from terminal to radio base station) and down-link transmission (DL=from radio base station to terminal).